1. Space Weathering on Phobos and Deimos
Carle M. Pieters
Brown Univ/SSERVI

2. Topics Why we suspect space weathering (SPWE) on Phobos & Deimos.
Review what we know of SPWE forms.
Lunar
Asteroids: S-type; Vesta
What do we know of Phobos/Deimos’ environment?
Summary and conjecture…..
CM Pieters SPWE

3. Phobos and Deimos variations are without obvious spectral features.
Rivkin et al (2002)
Phobos 2
Murchie et al., 1999
CM Pieters SPWE

4. All data are VIS-NIR scaled spectral reflectance.
Phobos and Deimos appear to have affinities with asteroid and lunar soils.
Reflectance Spectra of Exploration Targets:
Spectral range 450 – 2450 nm, scaled to unity
Remote measurement of spectral properties.
NEA
Main
Belt
Asteroids
All data are VIS-NIR scaled spectral reflectance.

5. Just what IS Space Weathering?
SPWE
Space weathering refers to an array of processes that measurably alter the character of surfaces with time as a result of exposure to the space environment.
CM Pieters SPWE

6. Just what IS Space Weathering?
MANY Processes!
Many Products!
CM Pieters SPWE

7. Just what IS Space Weathering?
S. Noble V1
V4 SPWE
CM Pieters SPWE

8. Just what IS Space Weathering?
Lunar-like SPWE: accumulation of npFe0 on regolith grains.
CM Pieters SPWE

9. Just what IS Space Weathering?
Vesta-like SPWE: contamination and mixing of exogenic (CC) materials.
CM Pieters SPWE

10. Just what IS Space Weathering?
Which processes are most active on
Phobos and Deimos?
CM Pieters SPWE

11. Observations & Ground Truth: The Moon
L. Keller, S. Noble et al.
1996-present
RELAB
With time, lunar soils…
Become darker across the visible
Have weaker absorption bands
Exhibit a ‘red’ sloped scaled nearIR continuum
CM Pieters SPWE

12. Observations & Ground Truth: The Moon
L. Keller, S. Noble et al.
1996-present
RELAB
With time, lunar soils…
Become darker across the visible
Have weaker absorption bands
Exhibit a ‘red’ sloped scaled nearIR continuum
CM Pieters SPWE

13. Evolution of thought… Old View
Space weathering occurs on the Moon, and….
agglutinates and glass are the cause of lunar optical alteration.
Since agglutinates are unlikely on asteroids, space weathering has no effect.
CM Pieters SPWE

14. Evolution of thought… Old View (incorrect!) Current Understanding
Space weathering occurs on the Moon, and….
agglutinates and glass are the cause of lunar optical alteration.
Since agglutinates are unlikely on asteroids, space weathering has no effect.
Current Understanding
Nano-phase opaques form on lunar grains by XX process.
During soil evolution, npFe0 accumulates on grains while Fe0 particles become larger within glass.
Absorptions weaken
Albedo reduced in the visible
Continuum lowers toward short wavelengths
Formation of npFe0 requires mobilization of Fe atoms…
CM Pieters SPWE

15. TEM Images of Lunar Soil Grains
npFe0 is observed as (sometimes multiple) deposits on rims
Fe0 particles on the interior of agglutinates are larger.
Plagioclase grain
L. Keller et al., 1996-present
Agglutinate grain

16. Optical properties and amount of npFe0 are directly related
Contaminant
0.005
0.02
0.07
Low FeO
0.13
0.19
0.30
High FeO
2.1
Lunar Samples: Noble et al., 2001
after Morris 1977
CM Pieters SPWE

17. Optical properties and amount of npFe0 are directly related
Contaminant
0.005
0.02
0.07
Low FeO
0.13
0.19
0.30
High FeO
2.1
Lunar Samples: Noble et al., 2001
Experimental: Noble et al., 2004; 2006
CM Pieters SPWE

18. Lunar-like SPWE Current Understanding
Nano-phase opaques form on lunar grains by XXprocess.
During soil evolution, npFe0 accumulates on grains while Fe0 particles become larger within glass.
Absorptions weaken
Albedo reduced in the visible
Continuum lowers toward short wavelengths
Formation of npFe0 on grains requires mobilization of Fe atoms…
CM Pieters SPWE

19. Is the same process likely or unlikely on asteroids?
CM Pieters SPWE

20. S-Asteroids are ABUNDANT!
Ordinary Chondrites are the most abundant meteorites.
S-type V-type C-type
40,000 asteroids
Sloan Digital Sky Survey
Asteroid Types
S-type
Distance from the Sun
Wavelength

21. NEO Observations & in-situ: Eros
S-Asteroid <–> Ordinary Chondrite? [Abundant]
NEAR
S-Asteroids have weaker absorption bands
….and a ‘red’ sloped continuum.
CM Pieters SPWE

22. NEO Observations & in-situ: Eros
S-Asteroid <–> Ordinary Chondrite?
[Abundant]
NEAR Results: Eros is a slightly (lunar-like) space weathered ordinary chondrite body….
[Trombka et al., 2000; Clark et al., 2001; McCoy et al. 2001]

23. Observations & Ground Truth: Itokawa
Hayabusa orbital and earth-based spectroscopy, suggested small amounts of lunar-like weathering for Itokawa.
Binzel et al. 2001, MaPS predict OC parent body.
CM Pieters SPWE

24. Observations & Ground Truth: Itokawa
Hayabusa returned regolith grains to Earth laboratories for analyses in 2010.
From Hayabusa orbital and earth-based spectroscopy, Itokawa shows small amounts of npFe0 weathering.
Furthermore, half of the grains studied contain np-opaque rims (both Fe and S). [Noguchi et al., 2011]
Demonstrated Itokawa is a LL Chondrite [Nakamura et al., 2011]
CM Pieters SPWE

25. Moon & NEA summary….
Prior to Dawn’s arrival at the main asteroid belt, a consistent story of space weathering had evolved:
Optical alteration is largely dominated by the accumulation of nano-phase opaques (npFe0 or npOpq) on the surface of regolith grains.
Both solar wind and micrometeorite processes are important (for the Moon, micro-impact vaporization & deposition may be common).
NEAs exhibit significantly less space weathering products than the Moon.
npFe0 is not the only phase to accumulate on regolith grains [composition of the host appears to be important].
CM Pieters SPWE

26. Moon & NEA summary…. BUT….. What of Vesta, a main-belt asteroid??
Prior to Dawn’s arrival at the main asteroid belt, a consistent story of space weathering had evolved:
Optical alteration is largely dominated by the accumulation of nano-phase opaques (npFe0 or npOpq) on the surface of regolith grains.
Both solar wind and micrometeorite processes are important (for the Moon, micro-impact vaporization & deposition may be common).
NEAs exhibit significantly less space weathering products than the Moon.
npFe0 is not the only phase to accumulate on regolith grains [composition of the host appears to be important].
BUT….. What of Vesta, a main-belt asteroid??

27. Observations & Ground Truth: Vesta
The HED meteorites (Howardites, Eucrites, Diogenites) have been associated with Vesta for decades
(McCord et al., 1970).
Telescopic data are well modeled by very fine-grained howardites, <25 µm.
[i.e., Space weathering is not needed?]
[Dawn instruments has confirmed the relation between Vesta and HEDs.]
CM Pieters SPWE

28. Observations & Ground Truth: Vesta
The HEDs (Howardites, Eucrites, Diogenites) have been associated with Vesta for decades (McCord et al., 1970).
Telescopic data are well modeled by fine-grained howardites. [i.e., Space weathering is not needed.]
[Dawn instruments has confirmed this relation between Vesta and HEDs.]
However, Vesta does exhibit space weathering !

29. Vesta Space Weathering
FC Image
Pieters et al.
Nature 2012
Prominent variations in band strength and albedo are observed, but no related variation in NIR continuum,  no npOpq.
CM Pieters SPWE

30. Two Key Questions: What IS causing the space weathering at Vesta?
Why has npFe0 NOT developed at Vesta?
CM Pieters SPWE

31. Darkening by small opaque particles: Scale is critical…
Impact darkening: 1-5 µm [Remobilization of opaques….]
npFe0 darkening: 1-5 nm [Vapor deposit or sputtering products]
Neutral color
“Red’ nearIR
CM Pieters SPWE

32. Two Key Questions: What IS causing the space weathering at Vesta?
Vesta exhibits a spectrally neutral absorbing species…
that is well mixed in the regolith*
and originates from contamination by low albedo asteroid debris [e.g. water-bearing CC**]
e.g. Zolensky et al 1996 MaPS
*See Pieters et al. and McCord et al., 2012 Nature
**See de Sanctis et al., 2012 ApJ and Prettyman et al.
2012, Science for OH/H2O discussion
CC clasts in HED meteorites

33. Two Key Questions: What IS causing the space weathering at Vesta?
Vesta exhibits a spectrally neutral absorbing species…
that is well mixed in the regolith*
and originates from contamination by low albedo asteroid debris [e.g. water-bearing CC**]
Why has npOpq NOT developed at Vesta?
Native Fe0 and FeS are almost absent on Vesta.
Pyroxene is more resistant to formation of npFe0.
Impact velocities are lower at Vesta.
Vesta is further from the Sun than Earth/Moon.
Rheasilvia is too recent.
Other…..
*See Pieters et al. and McCord et al., 2013 Nature
**See de Sanctis et al., 2012 ApJ and Prettyman et al.
2012, Science for OH/H2O discussion

34. Two Key Questions: What IS causing the space weathering at Vesta?
Vesta exhibits a spectrally neutral absorbing species…
that is well mixed in the regolith*
and originates from contamination by low albedo asteroid debris [e.g. water-bearing CC**]
Why has npOpq NOT developed at Vesta?
Native Fe0 and FeS are almost absent on Vesta.
Pyroxene is more resistant to formation of npFe0.
Impact velocities are lower at Vesta.
Vesta is further from the Sun than Earth/Moon.
Rheasilvia is too recent.
Other…..
*See Pieters et al. and McCord et al., 2013 Nature
**See de Sanctis et al., 2012 ApJ and Prettyman et al.
2012, Science for OH/H2O discussion

35. Key Lesson: All processes are active, but the relative effect varies with each body (location).
Contamination
CM Pieters SPWE

36. Summary….
The preponderance of nanophase opaques* (npOpq) on rims of regolith grains is a primary product of space weathering at 1 AU. However, cross-contamination and regolith mixing are now recognized as significant space weathering processes active on airless bodies. Estimations of the time scale for observed space weathering are non-unique and must directly address the processes and products involved. Simple parameters such as band strength or continuum slope cannot be used alone to measure the magnitude of space weathering.
CM Pieters SPWE
*can be Fe0, FeS, C, etc. …..

37. ….so what of Phobos? Phobos is OLD [Gys] Stickney is OLD [Gys]
Thus, the surface is old.
Best view of Phobos (Mars-side from HIRISE)
CM Pieters SPWE

38. All data are VIS-NIR scaled spectral reflectance.
Phobos and Deimos appear to have affinities with asteroid and lunar soils.
Reflectance Spectra of Exploration Targets:
Spectral range 450 – 2450 nm, scaled to unity
Remote measurement of spectral properties.
Asteroids
All data are VIS-NIR scaled spectral reflectance.

39. Phobos and Deimos spectra are…. without obvious features!
Rivkin et al (2002)
Fraeman et al., 2012; 2014
CM Pieters SPWE

40. ….but there is variability at Phobos
Phobos is OLD [Gys]
Stickney is OLD [Gys]
Thus, the surface is old.
But it is not homogeneous.
Old craters exist.
Fresh craters exist.
Boulders exist.
Fresh ‘red’ craters
Fresh ‘blue’ craters
Boulders
CM Pieters SPWE

41. ….so what of Phobos? Unlikely??
Our understanding of Mars two moons is very data limited [spectra, coverage, resolution].
However, Phobos and Deimos appear to share the same regolith optical properties.
If they both are captured asteroids, they must be the same type [D?] to produce the same regolith.
If they accreted from a major Mars impact …
Both likely started from highly processed material.
Their space weathering history is similar.
Unlikely??
CM Pieters SPWE

42. A SPWE ‘Story’* for Phobos
CM Pieters SPWE
*To be tested, revised…

43. A SPWE ‘Story’* for Phobos
After the Borealis impact event….
Phobos and Deimos accreted from anhydrous highly processed material (from Mars’ mantle?).
Minor heterogeneity was maintained, but neither evolved beyond a coherent rubble pile.
A regolith developed and was mixed with dust in the Mars environment.
With constant recycling, the regolith developed abundant lunar-like grain rims [abundant npOpq].
The local dynamic height configuration redistributes regolith across the moon in the patterns observed.
CM Pieters SPWE
*To be tested, revised…

44. Questions and Comments?

45. Dominant SPWE Process? [Comminution assumed for all]
The Moon [significant accumulation of npFe0]
Near-Earth Asteroids [accumulation of npOpq]
Vesta [contamination and regolith mixing]
Mercury [Uber-accumulation of npOpq?]
Phobos [? accumulation of npOpq + contamination]
Ceres [volatile mobility, + contamination, + dehydration, + regolith mixing?]
CM Pieters SPWE

46. Dominant Space Weathering Processes?
Differences are important!
Location
Temperature
Solar wind
Velocity of impactors
Dust environment
Composition
Presence/absence of iron, FeS….
Pyroxene/olivine
Water and volatiles
Other
Mass
Magnetic field
Close encounters
…..
CM Pieters SPWE